Effects of sediment parameters on the low frequency acoustic wave propagation in shallow water

The integral expression for acoustic field due to a point source in shallow waterwhile sediment is either a liquid or a solid is derived. The synthetic full waveforms are simulatedusing real axis integration and FFF method. The effects of the seabed sediment parametersand center frequency of the source on the low frequency acoustic wave propagation in shallowwater are investigated. The conclusion is that the wave groups received in far field are thoseof the mode waves of the source center frequency. The possibility for inversely deducing thecompressional and shear sound speeds of sediment using the least square optimum through themeasured group velocities of a selected mode at different frequencies is discussed.

The effects of bonding conditions on the propagation of acoustic waves along a cased borehole

Based on the wave equations in cylindrically layered structures and boundary conditions, the frequency equation for axisymmetric guided waves and the expression for sound fields in a cased borehole excited by a monopole or multipole source have been derived. The synthetic full waveforms excited by the monopole and dipole source are simulated using a real axis integration and FFT method. According to the axisymmetric guided wave modes, the synthetic full waveforms and the effects of the interface conditions on the sound field in a cased borehole have been analyzed and studied respectively. Numerical results indicate that it may be difficult to distinguish well bonded, poorly bonded or unbonded intermediate layer between the steel pipe and formation if only using a monopole source or dipole source. To properly estimate the case boundary conditions, a combination of monopole source logging with dipole source logging is suggested.

MIDDLE EAR MECHANISMS IN PROTECTION AGAINST NOISE INDUCED COCHLEAR DAMAGE

Quantitative estimation of sound transmission attenuation by different middle-ear protecting mechanisms was made with the use of a computer-based CM-cogram analyzing method (FFT of cochlear output in response to white noise) and other audiometric means. Strong reflex contraction of the middle-ear muscles in guinea pigs may cause a 5- 20 dB transmission attenuation in different frequency ranges. For impulse noise exposure, the presence of an effective reflex at the lashing of every impulse may reduce the post - exposure hearing threshold shift in 15 dB on an average.Increasing tension of the tympanic membrane by applying pressure to the ear- canal is also eflicacious.especially for frequencies below 500 Hz where attenuation of more than 10dB may result at a pressure difference of about 200mm H2O between the two sides of the membrane. Simulated opening of the eustachian tube helps only very insignificantly, with attenuation less than 5 dB. Destruction of the middle-ear structures ( drum perforation, ossicular disruption etc .) and fluid accumulation inside the tympanic cavity may buffer acoustic trauma to the cochlea to different extent depending on the amount of transmission attenuation.

INJURY TO THE MIDDLE EAR DUE TO HIGH-INTENSITY SOUND

In recent ten odd years, numerous experiments have been made on animals with high impulse noise and high - intensity continuous broadband noise, respectively. Subsequently a series of observations on human ears exposed to artillery fires have been conducted.